The present invention pertains to monitoring and measuring optical sources, and more particularly, to wavelength measurement of an optical source.
The widespread and exponential growth of the communication network systems during the past few years has provided for extensive proliferation of optics-based designs and systems. Due to the higher bandwidth and lower losses associated with optical signal transmission, optoelectronic systems are increasingly becoming the prevalent platforms for the implementation of many high speed network communication systems.
Because future high capacity services will have many different optical channels, methods that use the same equipment to monitor and measure the wavelength of multiple channels are preferable since they share the total cost among numerous channels.
The development of a fast and accurate wavelength measurement scheme is also desirable for a number of applications, such as distributed feedback lasers used in wavelength division multiplexed (WDM) communication systems or wavelength tunable lasers, wherein one or more wavelengths need be measured and monitored in order to adjust and stabilize the wavelength of the source.
A customary method of determining the wavelength of a light source is to split the signal into two paths and observe the interference pattern between the signal and a delayed version of itself, a technique commonly referred to as the Michelson interferometer technique. The wavelength of the input signal can then be obtained by carefully comparing the period of the zero-crossings of the resulting waveform with the waveform of a known standard.
The Michelson interferometer method requires a highly accurate laser wavelength reference and has moving mechanical parts that are quite bulky and necessitate fastidious alignment and calibration. Also, in the Michelson interferometer, the ratio of the index of refraction at the reference wavelength to the index of refraction at the unknown wavelength is a function of the ambient environment (such as humidity, gas content, temperature, etc.) which ultimately affects the accuracy of the measurement. In addition, erroneous results may be obtained if a modulated signal is present at the input of the Michelson interferometer.
Various other existing wavelength measurement techniques, such as the Static Fabry-Perot, the Frizeau interferometer and wavelength discriminators have been proposed, but they fail to provide a practical solution to alignment and detection problems commonly associated with accurate wavelength measurement. For instance, the Static Fabry-Perot interferometer has filters with repeated bandpass effect. The repeated bandpass phenomenon causes inaccuracy in the measurement since ascertaining which bandpass is being used in the repeated filter response is difficult. The Fizeau interferometer is only optimum for measuring light sources with low frequency modulation and suffers from poor sensitivity. Although wavelength discriminators offer a simple and cost effective solution to wavelength measurement, this comes at the cost of reduced performance and accuracy.
Hence, there is a need for a simple and accurate wavelength measuring technique. Preferably, the new method would eliminate the alignment and calibration requirements associated with mechanical parts used in many existing wavelength measurement set-ups and would be insensitive to changes in the testing environment. As well, the new technique would desirably be functionally precise when used for measuring modulated signals or in combination with optical filters.
The present invention addresses these key issues by providing a robust, fast, accurate, compact and potentially low cost method and apparatus for determining the wavelength of a particular light source which can be easily integrated into existing optoelectronics systems.
The invention can be used (a) as a means for determining the wavelength of a light beam, (b) as a wavemeter used for calibrating the wavelength steps of a tunable laser. A person skilled in the art may readily devise various other applications u sing the principles of th e invention.
The present invention involve s th e measurement of the time it takes for a pulse of light to propagate through an optical dispersive medium having known dispersion properties relative to wavelength. This propagation delay time can then be translated to wavelength using the calibrated properties of the dispersive medium.
In the wavelength measurement method using dispersion timing, the entire system consists of static components that are less bulky and less sensitive to inaccurate measurement than mechanically moving parts. This in turn offers great flexibility in design and provides for excellent structural integration in the design and implementation of a complex optical architecture. Also, another interesting aspect of the current invention is its design simplicity, providing for a robust, easily implementable and wavelength measurement system. Because this novel approach provides for precise optical monitoring of the wavelength, it becomes an optimal platform for wavelength stabilization to reduce cross-talk for use, for instance, in a WDM environment.
Yet another added advantage of the proposed method is that increasing the resolution only comes at the cost of reduced sensitivity, and not increased complexity, as with mechanical wavemeters.
Another advantage over the Michelson wavemeter is that, because it has no moving parts, calibration is easily maintained and variations of dispersion with temperature can also be calibrated.
Other aspects and advantageous features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.